Older Geologic Maps of Margarita Peak Area

See Flora of Margarita Peak: Physical Setting and Geology for an introduction to this page, and the later 2005 geologic mapping.

The following map gives the geology from the printed 1966 large-scale Santa Ana Sheet Geologic Map of California, and from the 2001 Preliminary Geologic Map 2001 of the Margarita Peak Quadrangle (5 MB).

For brevity on the map, the label "granite" refers to both granite and granodiorite.

Blue lines and blue labels correspond to the 1966 mapping (except for the blue line that follows 8S01 going roughly east-west along the top of the map). Red lines and red labels correspond to the 2001 mapping, which stops at about the middle of the above map.

The black diagonal lines indicate areas of disagreement between the two geologic maps.

The most surprising and significant difference is the western side of the parcel, where the 1966 mapping gives the rocks as Santiago Peak Volcanics. The 2001 mapping gives the rocks as fine-grained granodiorite in the northern two-thirds of the parcel, and granodiorite in the southern third. The 2005 mapping gives this area as quartz-bearing diorite, which seems to fit the rocks we observed in the field there.

The source of this discrepancy is probably the large-scale mapping in the 1966 version. There is an extensive area of Santiago Peak Volcanics to the west of this parcel; the eastern boundary was probably just not drawn accurately on the 1966 map.

The rest of the western half of the parcel is mapped as granodiorite in the 2001 mapping, but the 1966 mapping has a tongue of Bedford extending into the property. The 2005 mapping enlarges the area of Bedford around that tongue, and says that the granitic areas include significant Bedford rock. Most likely, the granodiorite and Bedford are quite intermixed here, making any given area hard to call.

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Copyright © 2007 by Tom Chester, James Dillane, Wayne Armstrong and Mike Peters
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Last update: 22 November 2007